(1) Summary of the Invention
The present invention relates to an improved adjustable diehead which mounts on an extrusion machine for forming plastic tubular parisons which can be subsequently blow molded to produce containers and the like. In particular, the present invention relates to an adjustment means for regulating the discharge orifice of the diehead so that the extruded parison has a uniform circumferential wall thickness.
(2) Prior Art
The prior art has described various types of means for adjusting the discharge orifice of a diehead. Illustrative are U.S. Pat. Nos. 3,386,132 to Fischer; 3,390,430 to Lynch et al; 3,535,739 to Mehnert; 3,564,653 to Sparks et al; 3,702,751 to Mehnert; 3,860,372 to Newman, Jr.; 3,895,900 to Herold et al; 4,047,868 to Kudo et al; and 4,465,449 to Hornbeck. Some of the dieheads are adjustable by adjustment means provided around an annular member providing a mandrel with a discharge orifice which presents safety problems if the adjustment is made during the operation of the extrusion machine. Some dieheads only provide for a gross or rough adjustment in the cross-sectional dimensions of the discharge orifice through vertical movement of a mandrel along its longitudinal axis in relation to an annular member.
In those extrusion machines which provide a means for radially adjusting the dimensions of discharge orifice, the safety of the operation is compromised. The discharge orifice is directly adjacent to and above the moveable components of the extrusion machine. This makes it difficult for an operator to reach behind the machine to gain access to the adjusting elements on the back side of the diehead which control the radial displacement of the angular member relative to the mandrel forming the discharge orifice. The dieheads are usually mounted in line to a common inlet port for the plastic which limits the space needed to adjust the discharge orifice of any particular diehead.
Those machines that provide only for vertical movement of the mandrel dimensions relative to an annular member as a means for adjusting the discharge orifice also have disadvantages. It is difficult to uniformly maintain the temperature of the extruded parison. Also experience has shown that it is difficult to uniformly regulate the pressure gradient of the molten plastic as it extrudes out the discharge orifice. Therefore, the extruded parison typically has a wall thickness that is not uniform even though the discharge orifice may be radially uniform. This can lead to problems in the wall thickness of the molded container being too thin in places which compromise strength and can cause the container to break or tear easily, or too thick in places which results in waste of plastic material.
Mehnert U.S. Pat. No. 3,702,751 describes the problems and overcomes the problems by using a biasing device which acts in concert with a biasing pin controlled by various adjustable means. Such a diehead provides good adjustment between the biasing device and the biasing pin; however, limited control of the annular member is provided around 360.degree. of the longitudinal axis of the mandrel. There is a need for a diehead which provides for precise radial adjustment of the position of the annular member relative to the mandrel around 360.degree. of the longitudinal axis of the mandrel, particularly from the front of the diehead facing the operator. U.S. Pat. No. 3,535,739 to Mehnert describes a more complicated diehead where the same type of control is used to move the annular member. Such a device is very expensive.